Pipelines can be formed from different steel types, including stainless steels as duplex, and martensitic chromium steels. However, all steels will corrode when exposed to water, especially sea water.
The coating of steel pipes is well known to prevent corrosion, but the coating is prone to damage, exposing the pipe directly to the water. Thus, a conventional method of assisting the corrosion-protection of steel pipes has been the addition of anodes, in particular sacrificial anodes, made from a metal which degrade preferentially to the pipe material.
The traditional technique for the connection of anodes to a pipeline has been to use pads welded directly to the pipeline by means of fillet welds. However, fillet welds create a stress concentration at the weld toe, leading to the possibility of hydrogen induced stress cracking (RISC), and thus failure of the pipeline.
GB2406367B describes an ‘anode bank’ pipe including a soft steel sleeve made from St255 or St355, and having at least 100 cm between the edge of the sleeve and an electrical conductor. The sleeve is provided in the form of two semi-circular profiles being preferably 6 m long placed over the load-bearing tube and secured together by a longitudinal weld. This sleeve is also welded to the tube at its ends. Further similar sleeve portions are preferably provided around the tube to provide additional strength to the connection between the sleeve and the tube to take account of the stress applied to the tube during use.
Thus, the application of each main sleeve discussed in GB2406367B requires welding operations, which still leads to the possibility of hydrogen induced stress cracking. Also, there is no assurance of the fixation of sleeve on the tube—hence the additional sleeve portions.
Also, although GB2406367B states that its pipe may be reeled onto a drum which allows for launching of a pipeline in use, the sleeve is described as being 2-10 m long, preferably 6 m long. Such a long sleeve is not easily bendable in its own right, and the problems of hydrogen induced stress cracking and the lack of integration between the sleeve(s) and the load-bearing tube would only be increased if such a pipe would ever be subjected to the bending required for the pipe to be reeled onto a drum. The length and nature of the sleeve do not suggest that this is possible in practice without affecting the integrity of the pipe and/or sleeve.